For example, a high intensity discharge lamp (an HID lamp) is used as a light source device of an image displaying optical apparatus, such as a liquid crystal projector or a DLP(™) projector.
In such a projector, light is divided into three primary colors R, G, and B by, for example, a dichroic prism, so that images are formed by spatial modulating elements corresponding to these three colors, and the R, G, and B image paths are re-synthesized by, for example, the dichroic prism, thereby displaying a color image.
According to another image forming method, light emitted from a light source passes through an R, G, B color filter which is rotated to sequentially generate R, G, and B light beams, and, in synchronization therewith, the spatial modulating elements are controlled to sequentially generate each of the three primary color images in a shifted time manner, thereby displaying a color image.
In order to turn on such a discharge lamp, while a voltage, called a no-load open circuit voltage, is applied to the discharge lamp, a high voltage is applied to the lamp to generate a breakdown in a discharge space of the lamp, so that a glow discharge and an arc discharge sequentially occur.
A high voltage can be applied to the lamp in either a method of applying a high voltage to main discharge electrodes of the lamp by using an igniter, that is, a serial trigger method, or a method of applying a high voltage to an auxiliary electrode provided so as not to come in contact with the discharge space in addition to the main discharge electrodes in the discharge space, that is, an external trigger method.
As compared with the serial trigger method, the external trigger method has an advantage in that, when a high-voltage generating unit including a high-voltage transformer is separated from a power supply circuit and is provided in the vicinity of a discharge lamp, it is possible to reduce the size, weight, noise, and manufacturing costs of a discharge lamp lighting device and to improve the stability of the discharge lamp lighting device.
On the other hand, discharge lamps in a normal lighting operation, are classified to two methods, a direct current driving method and an alternating current driving method.
In the direct current driving method, luminous flux from the lamp does not vary overtime as in a direct current. Therefore, the direct current driving method can be applied to either type of projectors.
In contrast, the alternating current driving method has an advantage in that it is possible to control the abrasion and growth of electrodes of a discharge lamp by using the degree of freedom, such as a polarity inverting frequency, that does not exist in the direct current driving method but it has problems which are attributed to polarity inversion.
In general, whenever the polarity inversion occurs performed in the alternating current driving method, for example, discontinuity of luminous flux from the lamp, an overshoot, and oscillation may occur. Therefore, in the projector driven by a time division system, asynchronism occurs between a timing when images are sequentially generated by time division and a polarity inversion timing when alternating current driving is performed on the lamp, that is, a displayed image is so excessively distorted at a peak frequency that it cannot be clearly recognized. Therefore, means for synchronizing the polarity inverting timing of the inverter with the rotation of color filters whose structure of the discharge lamp lighting device will be complicated, is needed.
Further, in the DLP-type projector, the brightness of the color of each pixel of a displayed image is controlled by an operational duty cycle ratio of the spatial modulating element in operation. In the alternating current driving method, even when the timing is synchronized, it is necessary to prevent light from being used or it is necessary to control the operation of each pixel of the spatial modulating element such that the variation in luminous flux is removed when a period in which a variation in luminous flux, such as an overshoot or oscillation, occurs at the time of polarity inversion, continues for a long time.
In the former case, the usage efficiency of light becomes low. In the latter case, it is difficult to control the spatial modulating elements of the projector.
In order to solve the above-mentioned problems of the method of driving the discharge lamp in the alternating current driving method, it is preferable to reduce the variation in luminous flux at the time of polarity inversion, but it is difficult to reduce the variation for the reasons set forth below.
That is, in the discharge lamp lighting device, it is necessary to reduce the variation in luminous flux when the polarity of a voltage applied to the lamp is inverted, to prevent an excessively large amount of current from flowing through the discharge lamp at the time of starting, to prevent the fade-out of discharge in the discharge lamp when a discharge mode is changed back from an arc discharge to a glow discharge, after changing to the glow discharge, and to reduce the ripple of a lamp current in a normal lighting mode.
When an excessively large amount of current flows through the lamp, the electrodes of the discharge lamp are damaged. When the fade-out of discharge occurs, the discharge lamp should be reignited. Thus, the life span of the lamp may be shortened. When a large ripple occurs in the lamp current in the normal lighting mode, the acoustic resonance of discharge occurs, thereby causing a flicker. For these reason, the above-mentioned requirements should be satisfied.
Japanese Unexamined Patent Application Publication No. 05-234685 discloses that, in order to improve the startability of a discharge lamp, a boost current flows through a discharge lamp, such as a metal halide lamp, at the time of starting by using a boost voltage circuit and charges stored in a boosting capacitor, while preventing an excessively large amount of peak current from flowing through it by, for example, a current restricting resistor, and after the start-up, the current restricting resistor is closed by a switch so that the boosting capacitor functions as a smoothing capacitor for reducing the ripple.
Japanese Unexamined Patent Application Publication No. 05-234685 also discloses that in order to an excessively large amount of peak current from flowing at the time of starting and to reduce the ripple of a lamp current in the normal lighting operation by the boost voltage circuit switch used as a switch for short-circuiting the current restricting resistor of the boosting capacitor.
However, at the time when the discharge lamp is driven by the alternating current driving method, the above art cannot be applied to reduce a variation in luminous flux when the polarity of a voltage applied to the lamp is inverted. In addition, when the discharge returns from the arc discharge to the glow discharge, the above-mentioned technology cannot effectively prevent the fade-out of discharge.
Further, a down-chopper circuit cannot respond to the high-speed change from a breakdown to an arc discharge since a smoothing capacitor should be charged, thereby causing the arc discharge to fail. In order to solve this problem, Japanese Unexamined Patent Application Publication No. 2000-123989 discloses that a switch is attached in a smoothing capacitor so as to decrease the capacitance of the smoothing capacitor when a breakdown by an igniter is transferred to an arc discharge from a glow discharge and then to increase the capacitance of the smoothing capacitor after or immediately after the discharge is transferred to the arc discharge, thereby reducing the ripple of a lamp current.
However, at the time when the discharge lamp is driven by the alternating current driving method, the art cannot reduce a variation in luminous flux when the polarity of a voltage applied to the lamp is inverted. In addition, when the discharge mode returns from the arc discharge to the glow discharge, the above-mentioned art cannot effectively prevent the fade-out of discharge.
Japanese Unexamined Patent Application Publication No. 2001-006895 discloses the capacitance of a smoothing capacitor of an output variable direct current source is increased when a discharge mode is transferred to an arc discharge after a glow discharge is completed, in order to prevent the fade-out of discharge when a high-pressure mercury lamp returns from the arc discharge to the glow discharge, to prevent an excessively large amount of current from flowing at the time of starting, and to reduce the ripple of a lamp current in a normal lighting mode.
However, at the time when the discharge lamp is driven by the alternating current driving method, this art cannot reduce a variation in luminous flux when the polarity of a voltage applied to the lamp is inverted.
Further, Japanese Unexamined Patent Application Publication No. 2002-117990 discloses the capacitance of a smoothing capacitor immediately is decreased after the lighting of a discharge lamp is transferred from a glow discharge mode to an arc discharge, and the capacitance of the smoothing capacitor is increased when the arc discharge is stably maintained, in order to prevent the fade-out of discharge when the discharge lamp returns from the arc discharge mode to the glow discharge mode and to reduce the ripple of a lamp current in a normal lighting mode.
However, at the time when the discharge lamp is started by the alternating current driving method, this art cannot reduce a variation in luminous flux when the polarity of a voltage applied to the lamp is inverted. In addition, the above-mentioned art cannot effectively prevent an excessively large amount of current from flowing through the discharge lamp at the time of starting.